Chapter 6.1 What You Will Learn • Earth’s interior can be divided into layers based on chemical composition and physical properties. • Scientists use seismic waves to study Earth’s interior. • Continents are drifting apart from each other now and have done so in the past. The Layers of the Earth • Earth is made of several layers. • The materials in each layer have distinct properties. • Earth’s layers can be described in terms of their chemical composition or physical properties. The Compositional Layers of Earth • Earth is divided into three compositional layers. • At Earth’s center, the dense metallic core is made mainly of the metal iron. • The dense, thick middle layer is the mantle. • The mantle is made up largely of silicon, oxygen, and magnesium. • The surface layer, or crust, is composed mostly of silicon, oxygen, and aluminum. Continental and Oceanic Crust • There are two types of crust. • Continental crust is thicker than oceanic crust. • Both types are made mainly of the elements oxygen, silicon, and aluminum. • But oceanic crust has almost twice as much iron, calcium, and magnesium as continental crust does. • These three elements form minerals that are denser than the minerals in continental crust. • These dense minerals make the thin oceanic crust heavier than the thicker continental crust. The Physical Structure of Earth • Earth is divided into five layers based on physical properties. • Earth’s outer layer is the lithosphere, which is a cool, rigid layer that includes the crust and the upper part of the mantle. • The lithosphere is divided into pieces called tectonic plates. • Below the lithosphere is the asthenosphere, which is a layer of the mantle that is made of very slowflowing solid rock. • Tectonic plates move on top of the asthenosphere. • Below the asthenosphere is the mesosphere, which is the lower part of the mantle. • The mesosphere flows even more slowly than the asthenosphere. • Earth’s core has two layers. • The outer core is a layer of liquid iron and nickel. • At Earth’s center is the solid inner core. This layer is made mostly of nickel and iron. • The inner core is very hot, but it is solid because it is under enormous pressure. Mapping Earth’s Interior • Scientists have learned about Earth’s interior by studying earthquakes. • An earthquake produces vibrations called seismic waves. • Seismic waves travel through Earth at various speeds. • Machines called seismometers measure the time seismic waves take to travel various distances from an earthquake’s center. • Scientists use these distances and times to calculate the density and thickness of Earth’s layers. • The speed of seismic waves is affected by the type of material that the waves are traveling through. • For example, some types of waves can travel through rock but not through liquids. • Scientists have found that such waves do not reach seismometers on the side of Earth opposite the earthquake. • So, part of Earth’s interior must be liquid. • This liquid layer is the outer core. Continental Drift • Continental drift is the idea that a single large landmass broke up into smaller landmasses to form the continents, which then drifted to their present locations. • This idea explains how the continents seem to fit together. • Continental drift also explains why fossils of the same plant and animal species are found on continents that are far away from each other. • Many of these ancient species could not have crossed an ocean. • The locations of mountain ranges and similar types of rock also support continental drift. • Scientists have used rock and fossil evidence to reconstruct past patterns of climate regions. • The distribution of these ancient climatic zones supports the idea of continental drift, too. The Breakup of Pangaea • Alfred Wegener, the scientist who proposed the theory of continental drift, proposed that the large continent of Pangaea gave rise to today’s continents. • Scientists have determined that Pangaea existed about 245 million years ago. • Pangaea split into two continents—Laurasia and Gondwana—about 135 years ago. • These two continents then split into the continents we know today. • These continents slowly drifted to their present positions. Sea-Floor Spreading • Evidence for continental drift lies on the sea floor. • A chain of submerged mountains runs through the center of the Atlantic Ocean. • This mountain chain is part of a worldwide system of mid-ocean ridges. • Mid-ocean ridges show patterns of magnetism. • The pattern on one side of a ridge is the mirror image of the pattern on the other. • The magnetism of rocks aligns with Earth’s magnetic field as it was when the rocks formed. • Throughout Earth’s history, the north and south magnetic poles have changed place many times. • The process by which the poles change places is called magnetic reversal. • As rock forms from magma, minerals that contain iron form. • Some of these minerals are magnetic and act like compasses. • They form so that their magnetic fields align with the magnetic fields on Earth. • When the molten rock cools, these tiny compasses are locked into position in the rock. • After Earth’s magnetic field reverses, new magnetic minerals that align in the opposite direction form. • At a mid-ocean ridge, magma rises through fractures in the sea floor. • As magma cools, it forms new rock. • As the new rock forms, the older rock gets pulled away from the mid-ocean ridge. • The process by which new sea floor forms as old sea floor is pulled away is called sea-floor spreading. • The record of magnetic reversals on the sea floor provides evidence that the continents are moving. • Sea-floor spreading is one process that moves continents.